In the preparation of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. (YBCO) thick and thin films, choice of substrate is a key factor. Chemical non-reactivity between the substrate and YBCO is the most crucial for obtaining a superconducting high T.sub.c(o) film. Besides, for microwave applications, the substrate should have very low dielectric constant (=10) and the loss factor (10.sup.-4 -10.sup.-5) at GHz frequencies (Brandle and Fratello, J. Mater. Res., 5, 2160 (1990)). For epitaxial growth of the film, lattice matching is also important. MgO is the most widely reported substrate material which satisfies the above substrate characteristics reasonably well for YBCO films. However, MgO do form an interlayer of Ba salt at the YBCO--MgO interface if the temperature of processing is above 700.degree. C. (Koinuma et al., Jpn J. Appl. Phys. 27, L1216 (1988), Cheung and Ruckenstein, J. Mater. Res. 4, 1 (1989) and Preng et al., Supercond. Sci. Technol. 3, 233 (1990)). At higher temperatures (&gt;700.degree. C.) chemical interdiffusion of the cation across the film boundary in YBCO--MgO is quite prominent (Hu et al., "Advances in Superconductivity", (Proc. 1st Inter. Symp. on Supercond. ISS 88. Aug. 28-31, 1988, Nagoya, Japan), Edited by K. Kitazawa and I. Ishiguro, Springer-Verlag, Tokyo, p. 647 (1989) and Li et al., J. Mater. Sci., 26, 1057 (1991)). In particular, in the case of thick film preparation of YBCO using MgO substrate, where the processing temperature required is well above 900.degree. C., the Ba salt formation and cation interdiffusion reduce the superconducting transition t.sub.c(o) of YBCO thick film very appreciably (Bansal, Mater. Lett., 13, 7 (1992)). Other commercially available substrates such as Si, SiO.sub.2, Al.sub.2 O.sub.3, SrTiO.sub.3, GaAlO.sub.3, LaAlO.sub.3 etc. either chemically react with YBCO or have a high dielectric constant and loss factor, which makes them unsuitable or less attractive as substrates for YBCO at microwave frequencies. The main object of the present invention is to produce a process for the preparation of ceramic substrates of REBa.sub.2 MO.sub.6 (where RE=Rare Earth metals--Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and M=Nb, Sb, Sn, Hf, Zr) which are chemically compatible with YBCO and have very low dielectric constant and loss factor making them suitable for microwave applications.